Researchers modeled nutrient flow in the oceans under several greenhouse gas emissions scenarios. And they found that, by the year 2100, nutrients at the ocean's surface may dwindle, leaving fewer leftovers to float down to organisms below. The result could be a massive die-off of life on the seafloor, like sea cucumbers, starfish, urchins and worms. Under a severe emissions scenario, the loss of marine life globally would equal the biomass of the entire human race.

One bright spot in the deep darkness: the exotic tube worms and giant clams that thrive at hydrothermal vents don't need surface nutrients to survive. But plenty of other species do, the researchers say—and we don't even know much of what's down there. This study makes one thing clear: when it comes to climate change and the oceans, we're already in deep.